Sound reproduction system



' May 2, 1961 D. KLEIS ETAL 2,982,821

SOUND REPRODUCTION SYSTEM Filed Nov. 21. 1955 INVENTORS DERK KLEIS ROELOF VERMEULEN BY H 1 $4 a AGEN United States Patent I SOUND REPRODUCTION SYSTEM Derk Kleis and Roelof Vermeulen, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Nov. 21,1955, Ser. No. 548,210 Claims priority, application Netherlands Nov. 30, 1954 3 Claims. (Cl. 179--100.l)

sound is to be understood to mean sound reaching the listener substantially from all directions with approximately the same intensity and approximately the same time delay. However, it is possible to obtain localization of the sound in the diflfuse sound picture at given loudspeakers, for example, in the case of a soloist playing together with an orchestra.

Such a method has the advantage that the presence or absence and the nature of the diffuse sound can be determined in the initial recording of the sound, so that this need not occur at the point of audio reproduction. This advantage is emphasized by the fact that a given record itself, or copies thereof, is usually to be reproduced at a large number of places.

Such a method may, for example, be employed in motion picture theatres, where the sound recordings synchronized with the picture film are used on one film strip. The loudspeakers reproducing the signals recorded in the stereophonic or non-stereophonic. sound tracks are generally arranged behind the projection screen of a theatre and radiate direct sound. Said loudspeakers will hereinafter be termed direct loudspeakers," whereas the remaining loudspeakers will be termed indirect loudspeakers.

In order to obviate departure from true audio reproduction on account of strong directivity of direct sound radiation, it is known to emit the sound from the indirect loudspeakers to the listener with the same time delay. This permits a certain degree of diffusion of the reproduced sound to be obtained, with the result of approaching a reality where reflections from the walls likewise contribute considerably to the quality of the sound.

However, this method involves serious difiiculties. As a matter of fact, it is sometimes desirable to reproduce sound of a given nature, for example, music, through indirect loudspeakers withoutlocalization of the sound picture, so that the overall reproduction of sound is the same to the listener in all directions. In the case of background music, for example, for motion picture films, where there is no direct relationship between auditory and visual perception, there is no need for localization.

However, this method suffers from a limitation in that a listener will first receive the sound from the nearest loudspeaker, the sound of which has the shortest traveling time and the highest intensity, so that unintentional localization of the sound picture occurs at said nearest loudspeaker.

The uniform time delay of the signals feeding the indirect loudspeakers in the aforesaid method may be chosen arbitrarily within given limits. It has, however,

proved necessary to fulfill the condition that, on the one hand, indirect sound should not reach some listener in the theatre earlier than direct sound, since otherwise erroneous localization of the sound picture would occur and, on the other hand the difference between indirect and direct sound should not exceed a given time limit of approximately 50 milliseconds, since otherwise two separate signals are perceived (echo effect).

It has proved more difficult to fulfil both conditions as the size of the theatre increases. This is due to the fact that, on account of the distance of the loudspeaker from the listener, the time delay will increase as the theatre space increases and may as such counteract correct perception at any desired point. This will be explained with reference to the accompanying drawing.

Applicants have recognized that said difficul-ties can be overcome also in enclosed areas where the time delay is of the same order as, or exceeds that occurring in recording direct and indirect sound. v

In the sound reproduction method of the present invention, a single additional sound track is scanned by a number of reproducing elements or heads positioned one behind the other in a longitudinal direction of said track, and the signal of each reproducing element is supplied, if desired, with dilferent intensity and/or timbre, to given loudspeakers or loudspeaker groups. The invention permits several loudspeakers or loudspeaker groups to be supplied with different time delays, thus meeting the aforesaid disadvantages by compensation of the traveling time differences of the several indirect loudspeakers and, if desired, also of the direct loudspeakers if the latter are also used in sound reproduction. In this manner it is insured that the loudspeakers produce diffuse sound of which substantially no direction can be localized, so that the reproduction of background music can be closely'approached. An example is the-case where a soloist plays together with an orchestra. In this instance, the indirect loudspeakers reproduce the orchestra sounds as diffuse sound with all the advantages accruing from the invention, whereas the soloists sounds are localized by the direct loudspeaker. From this it is evident that the method of the present invention permits diffuse radiation of sound by means of indirect loudspeakers while direct loudspeakers radiate a different sound. I

If, as in the case of background music, all the sound tracks, inclusive of the additional sound track, comprise one and the same signal or, if they comprise at least one common signal, the present invention permits the delay of the signals, which are reproduced by the indirect loudspeakers, to be properly chosen in order to effect localization at the direct loudspeakers. In one example thereof, a voice is reproduced by all the loudspeakers, while localization at the direct loudspeakers is wanted.

In the method of the present invention, in order to insure localization of the virtual sound source at the loudspeaker or loudspeaker group not supplied from the additional sound track, the reproduction of the common signal by means of the first reproducing element scanning the additional track is delayed sufficiently, preferably from 5 to 50 milliseconds, relative to the reproduction of the other tracks. This is based on the fact that, for a delay of less than 5 milliseconds, the effects, in regard to localization of time differences and intensity difierences have the same influence (the effect of a millisecond delay having approximately the same influence as an intensity decrease of 5 db) and the effects are additive, whereas for a delay of more than 5 milliseconds the direction of the first arriving sound dominates and may be practically influenced only by very considerable intensity differences.

The time delay is obtainable by making allowance for it in producing the sound record, or by making provision that the first reproducing element occupies the proper position along the initial track.

In order that the invention may be readily carried into effect it will now be described with reference to the accompanying drawing, given by way of example, in which:

Fig. 1 is a schematic diagram of an enclosed area around which both direct and indirect loudspeakers are distributed in accordance with the present invention; and

Fig. 2 is a schematic diagram of part of a film record and several reproducing heads in accordance with the present invention.

Fig. 1 represents an enclosed area 1 in which three direct loudspeakers 2, 3 and 4, are positioned along one side. These loudspeakers produce three associated stereophonic sound signals. Loudspeakers 5, 6 and 7 producing indirect sound are positioned along the other sides. The loudspeakers 5 and 6 are directly interconnected as shown.

In order to reproduce sound in the area 1 the loudspeakers or loudspeaker groups are connected to reproducing elements for scanning a sound record which comprises three stereophonic sound tracks from which loudspeakers 2, 3 and 4 are supplied. Besides, there is a single additional sound track comprising a signal which is different from the signal of the remaining tracks or is already present in these tracks.

In connection with a given size of the area 1, a listener at A will perceive direct sound after, say, 70 milliseconds. This corresponds to a distance of approximately 24 meters and necessitates that indirect sound from the loudspeaker 7 be delayed at least 70 milliseconds if the point of the sound picture is not to be localized at A. If the same time delay is also allotted to the loudspeakers 5, a listener at B, who is at about the same distance from the loudspeaker 3 as from the loudspeakers 5, will perceive the indirect sound as an echo of the'direct sound. The mitigation of this inconvenience will be explained with reference to Fig. 2.

Fig. 2 is a schematic diagram of part of a film record and several reproducing heads in accordance with the present invention. A sound record 8 is provided with four sound tracks 9, l0, l1 and 12, the first three of which represent stereophonic signals, and the last of which, 12, designates an additional track. These sound tracks are scanned by a number of reproducing heads 2', 3', 4, 5', 6' and 7' which are numbered to correspond to the reference numerals of their associated loudspeakers. The record 8 may be a magnetic tape and the rearbitrary location with indirect and direct sound and t0.

fulfill the aforesaid conditions.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

What is claimed is:

l. A sound reproduction system for an enclosed area comprising a sound record having a plurality of initial sound tracks recorded thereon, an additional sound track recorded on said sound record, means for scanning said initial tracks comprising a reproducing head positioned in operative relation with each of said initial tracks,

means for scanning said additional track comprising a pluralityof reproducing heads positioned in sequential relation along said additional track and in operative relation with the said additional track, a plurality of loudspeakers positioned along the boundaries of said area for reproducing in a diffuse manner the sound recorded on said sound record, means for connecting each of the initial track reproducing heads to each of a predetermined group of loudspeakers for producing direct sound, and means for connecting the additional track reproducing heads to the remaining loudspeakers for producing indirect sound, said reproducing heads being so positioned with respect to each other that the reproduction of indirect sound by said remaining loudspeakers is delayed by predetermined periods of time relative to the reproduction of direct sound by said group of loudspeakers.

2. A sound reproduction system for an enclosed area comprising a sound record having a plurality of initial sound tracks recorded thereon, an additional sound track recorded on said sound record, means for scanning said initial tracks comprising a reproducing head positioned in operative relation with each of said initial tracks, means for scanning said additional track comprising a plurality of reproducing heads positioned in sequential relation along said additional track and in operative relation with the said additional track, a plurality of loudspeakers positioned along the boundaries of said area for reproducing in a diffuse manner the sound recorded on said sound record, means for connecting each of the initial track reproducing heads to each of a predetermined group of loudspeakers for producing direct sound, and means for connecting the additional track reproducing heads to the remaining loudspeakers for producing indirect sound, said reproducing heads being so positioned with respect to each other that the reproduction of indirect sound by said remaining loudspeakers is delayed by predetermined periods of time relative to the reproduction of direct sound by said group of loudspeakers in accordance with the distance of said remaining loudspeakers from the said group of loudspeakers.

3. A sound reproduction system for an enclosed area comprising a sound record having a plurality of initial sound tracks recorded thereon, an additional sound track recorded on said sound record, a common signal being recorded on said tracks, means for scanning said initial tracks comprising a reproducing head positioned in operative relation with each of said initial tracks, means for scanning said additional track comprising a plurality of reproducing heads positioned in sequential relation along said additional track and in operative relation with the said additional track, a plurality of loudspeakers positioned along the boundaries of said area for reproducing in a diffuse manner the sound recorded on said sound record, and means for connecting each of said reproducing heads to selected ones of said loudspeakers, said reproducing heads being so positioned with respect to each other that the reproduction of said common signal by the loudspeaker connected to the first of the sequentially positioned reproducing heads to scan said additional track is delayed by a predetermined period of time relative to the reproduction of sound by the remaining loudspeakers and the virtual sound source is localized at the loudspeakers connected to the reproducing heads of said initial tracks.

Bogert Ian. 7, 1958 

